I'd always say wire brush it until it's shiny.
I wouldn't trust any chemical to remove all of the rust completely....and the only way you'll know it has is by wire brushing it anyway.
There is one way to look at rust though. if no air can get to it, it won't develop further. So if you do leave a bit there and the paint keeps it perfectly sealed in, you're unlikely to have a problem.
The problem is, in some situations the paint just doesn't stay put long enough and often rust makes the metal porous to the other side of the panel pretty quickly. So just pop a wire brush on the angle grinder and keep going until you smell your fingers buuuuurn!